Ink jet inks containing block copolymers of polyethylene oxide and polypropylene

ABSTRACT

A liquid ink jet ink wherein said ink is non-gelling and comprises a carrier, a pigment and a solid block copolymer of ethylene oxide and propylene oxide having a structure (A) and (B) defined herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation-In-Part of application Ser. No. 08/573,712, filed Dec. 18, 1995, now abandoned.

FIELD OF THE INVENTION

This invention relates to the field of ink jet printing.

BACKGROUND OF THE INVENTION

Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.

Pigment based inks have been gaining in popularity as a means of addressing these limitations. In pigment-based inks, the colorant exists as discrete particles. The problem is that these inks interact with specially coated image receiving substrates, such as transparent films used for overhead projection and the glossy papers and opaque white films used for high quality graphics, to produce cracked images.

U.S. Pat. No. 5,462,591, by Karandikar et al, discloses an aqueous phase change ink that contains dyes and block copolymers of polyethylene oxide and polypropylene oxides as thermogelling components that causes the ink to gel when its temperature is increased to its thermoinversion point. No pigments are included in these inks.

SUMMARY OF THE INVENTION

The present invention provides ink jet inks, comprising a liquid ink jet ink wherein said ink is non-gelling and comprises a carrier, a pigment and a solid block copolymer of ethylene oxide and propylene oxide having a structure selected from the group consisting of: ##STR1## having a number average molecular weight of 4000 to 15,000 and an ethylene oxide content of 80% by weight; and ##STR2## having a number average molecular weight of 4000 to 9000 and an ethylene oxide content of 80% by weight.

Images printed using these ink jet inks have much improved quality. In many cases cracking is entirely eliminated. The inks do not gel and do not have any thermoinversion characteristics.

DETAILED DESCRIPTION OF THE INVENTION

As stated above the useful copolymers consisted of blocks of ethylene oxide and propylene oxide. The structure may be either linear triblock (ABA or BAB) morphology in which A represents polyethylene oxide and B the polypropylene oxide.

Solid block copolymers (A) and (B) having the defined molecular weights, and the blocks of polyethylene oxide and polypropylene oxide are commercially available from BASF Corporation under the name Pluronic® surfactants. Block copolymer concentration in the inks is most useful from 0.2 to 5 weight percent, based on the total weight of the ink composition. Concentration below 0.2 weight percent have no effects. At concentrations higher than 5 weight percent image quality deteriorates.

Examples of useful block copolymers, together with their respective Pluronic® trade designations, number average molecular weights, and ethylene oxide contents are presented below. Examples of ABA block copolymers according to structure I are:

    ______________________________________                                         Pluronic ®        Ethylene Oxide                                             Designation MW Weight %                                                      ______________________________________                                         F38            4k     80                                                         F68 8k 80                                                                      F108 14K .sup.  80                                                           ______________________________________                                    

Examples of BAB block copolymers according to structure II are:

    ______________________________________                                         Pluronic ®        Ethylene Oxide                                             Designation MW Weight %                                                      ______________________________________                                         10R8           5K     80                                                         17R8 7K 80                                                                     25R8 9K 80                                                                   ______________________________________                                    

The process of preparing inks from pigments commonly involves two steps: (a) a dispersing or milling step to break up the pigment to the primary particle, and (b) dilution step in which the dispersed pigment concentrate is diluted with a carrier and other addenda to a working strength ink. In the milling step, the pigment is usually suspended in a carrier (typically the same carrier as that in the finished ink) along with rigid, inert milling media. Optionally, the block copolymers defined for this invention may be added to the mill grind. Mechanical energy is supplied to this pigment dispersion, and the collisions between the milling media and the pigment cause the pigment to deaggregate into its primary particles. A dispersant or stabilizer, or both, is commonly added to the pigment dispersion to facilitate the deaggregation of the raw pigment, to maintain colloidal particle stability, and to retard particle reagglomeration and settling.

There are many different types of materials which may be used as milling media, such as glasses, ceramics, metals, and plastics. In a preferred embodiment, the grinding media can comprise particles, preferably substantially spherical in shape, e.g., beads, consisting essentially of a polymeric resin.

In general, polymeric resins suitable for use as milling media are chemically and physically inert, substantially free of metals, solvent and monomers, and of sufficient hardness and friability to enable them to avoid being chipped or crushed during milling. Suitable polymeric resins include crosslinked polystyrenes, such as polystyrene crosslinked with divinylbenzene, styrene copolymers, polyacrylates such as poly(methyl methylacrylate), polycarbonates, polyacetals, such as Derlin™, vinyl chloride polymers and copolymers, polyurethanes, polyamides, poly(tetrafluoroethylenes), e.g., Teflon™, and other fluoropolymers, high density polyethylenes, polypropylenes, cellulose ethers and esters such as cellulose acetate, poly(hydroxyethylmethacrylate), poly(hydroxyethyl acrylate), silicone containing polymers such as polysiloxanes and the like. The polymer can be biodegradable. Exemplary biodegradable polymers include poly(lactides), poly(glycolids) copolymers of lactides and glycolide, polyanhydrides, poly(imino carbonates), poly(N-acylhydroxyproline) esters, poly(N-palmitoyl hydroxyprolino) esters, ethylene-vinyl acetate copolymers, poly(orthoesters), poly(caprolactones), and poly(phosphazenes). The polymeric resin can have a density from 0.9 to 3.0 g/cm³. Higher density resins are preferred inasmuch as it is believed that these provide more efficient particle size reduction. Most preferred are crosslinked or uncrosslinked polymeric media based on styrene.

Milling can take place in any suitable grinding mill. Suitable mills include an airjet mill, a roller mill, a ball mill, an attritor mill and a bead mill. A high speed mill is preferred.

By high speed mill we mean milling devices capable of accelerating milling media to velocities greater than about 5 meters per second. The mill can contain a rotating shaft with one or more impellers. In such a mill the velocity imparted to the media is approximately equal to the peripheral velocity of the impeller, which is the product of the impeller revolutions per minute, π, and the impeller diameter. Sufficient milling media velocity is achieved, for example, in Cowles-type saw tooth impeller having a diameter of 40 mm when operated at 9,000 rpm. The preferred proportions of the milling media, the pigment, the liquid dispersion medium and dispersant can vary within wide limits and depends, for example, upon the particular material selected and the size and density of the milling media etc. The process can be carried out in a continuous or batch mode.

Batch Milling

A slurry of <100 μm milling media, liquid, pigment and dispersant is prepared using simple mixing. This slurry may be milled in conventional high energy batch milling processes such as high speed attritor mills, vibratory mills, ball mills, etc. This slurry is milled for a predetermined length of time to allow comminution of the active material to a minimum particle size. After milling is complete, the dispersion of active material is separated from the grinding media by a simple sieving or filtration.

Continuous Media Recirculation Milling

A slurry of <100 μm milling media, liquid, pigment and dispersant may be continuously recirculated from a holding vessel through a conventional media mill which has a media separator screen adjusted to >100 μm to allow free passage of the media throughout the circuit. After milling is complete, the dispersion of active material is separated from the grinding media by simple sieving or filtration.

With either of the above modes the preferred amounts and ratios of the ingredients of the mill grind will vary widely depending upon the specific materials and the intended applications. The contents of the milling mixture comprise the mill grind and the milling media. The mill grind comprises pigment, dispersant and a liquid carrier such as water. For aqueous ink jet inks, the pigment is usually present in the mill grind at 1 to 50 weight %, excluding the milling media. The weight ratio of pigment to dispersant is 20:1 to 1:2. The high speed mill is a high agitation device, such as those manufactured by Morehouse-Cowles, Hockmeyer et al.

The dispersant is another important ingredient in the mill grind. Preferred dispersants for aqueous ink jet inks include sodium dodecyl sulfate, acrylic and styrene-acrylic copolymers, such as those disclosed in U.S. Pat. Nos. 5,085,698 and 5,172,133, and sulfonated polyesters and styrenics, such as those disclosed in U.S. Pat. No. 4,597,794. Other patents referred to above in connection with pigment availability also disclose a wide variety of dispersant to select from. The dispersant used in the examples is sodium N-methyl-N-oleoyl taurate (OMT).

The milling time can vary widely and depends upon the pigment, mechanical means and residence conditions selected, the initial and desired final particle size, etc. For aqueous mill grinds using the preferred pigments, dispersants, and milling media described above, milling times will typically range from 1 to 100 hours. The milled pigment concentrate is preferably separated from the milling media by filtration.

In the present invention, any of the known pigments can be used. Pigments can be selected from those disclosed, for example, in U.S. Pat. Nos. 5,026,427; 5,085,698; 5,141,556; 5,160,370 and 5,169,436. The exact choice of pigment will depend upon the specific color reproduction and image stability requirements of the printer and application. For four-color printers combination of cyan, magenta, yellow, and black (CMYK) pigments should be used. An exemplary four color set may be copper phthalocyanine (pigment blue 15), quinacridone magenta (pigment red 122), pigment yellow 138 and carbon black (pigment black 7).

The aqueous carrier medium is water or a mixture of water and at least one water soluble co-solvent. Selection of a suitable mixture depends on requirements of the specific application, such as desired surface tension and viscosity, the selected pigment, drying time of the pigmented ink jet ink, and the type of paper onto which the ink will be printed. Representative examples of water-soluble co-solvents that may be selected include (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propy alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; (3) ethers, such as tetrahydrofuran and dioxane; (4) esters, such as ethyl acetate, ethyl lactate, ethylene carbonate and propylene carbonate; (5) polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol 1,2,6-hexanetriol and thioglycol; (6) lower alkyl mono- or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl (or -ethyl) ether, diethylene glycol mono-methyl (or -ethyl) ether, propylene glycol mono-methyl (or -ethyl) ether, triethylene glycol mono-methyl (or -ethyl) ether and diethylene glycol di-methyl (or -ethyl) ether; (7) nitrogen containing cyclic compounds, such as pyrrolidone, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone; and (8) sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone.

Ink Preparation

In general it is desirable to make the pigmented ink jet ink in the form of a concentrated mill grind, which is subsequently diluted to the appropriate concentration for use in the ink jet printing system. This technique permits preparation of a greater quantity of pigmented ink from the equipment. If the mill grind was made in a solvent, it is diluted with water and optionally other solvents to the appropriate concentration. If it was made in water, it is diluted with either additional water or water miscible solvents to make a mill grind of the desired concentration. By dilution, the ink is adjusted to the desired viscosity, color, hue, saturation density, and print area coverage for the particular application.

In the case of organic pigments, the ink may contain up to approximately 30% pigment by weight, but will generally be in the range of approximately 0.5 to 10%, preferably approximately 0.1 to 5%, by weight of the total ink composition for most thermal ink jet printing applications. If an inorganic pigment is selected, the ink will tend to contain higher weight percentages of pigment than with comparable inks employing organic pigments, and may be as high as approximately 75% in come cases, since inorganic pigments generally have higher specific gravities than organic pigments.

The amount of aqueous carrier medium is in the range of approximately 70 to 98 weight %, preferably approximately 90 to 98 weight %, based on the total weight of the ink. A mixture of water and a polyhydric alcohol, such as diethylene glycol, is preferred as the aqueous carrier medium. In the case of a mixture of water and diethylene glycol, the aqueous carrier medium usually contains from about 30% water/70% diethylene glycol to about 95% water/5% diethylene glycol. The preferred ratios are approximately 60% water/40% diethylene glycol to about 95% water/5% diethylene glycol. Percentages are based on the total weight of the aqueous carrier medium.

Block copolymers defined for this invention are added in a concentration of 0.2 to 5 weight percent as previously stated if not already included in the mill grind.

Jet velocity, separation length of the droplets, drop size and stream stability are greatly affected by the surface tension and the viscosity of the ink. Pigmented ink jet inks suitable for use with ink jet printing systems should have a surface tension in the range of about 20 dynes/cm to about 60 dynes/cm and, more preferably, in the range 30 dynes/cm to about 50 dynes/cm. Control of surface tensions in aqueous inks is accomplished by additions of small amounts of surfactants. The level of surfactants to be used can be determined through simple trial and error experiments. Anionic and cationic surfactants may be selected from those disclosed in U.S. Pat. Nos. 5,324,349; 4,156,616 and 5,279,654 as well as many other surfactants known in the ink jet ink art. Commercial surfactants include the Surfynols® from Air Products; the Zonyls® from DuPont and the Fluorads® from 3M.

Acceptable viscosities, as determined using Brookfield apparatus and related methods are no greater than 20 centipoise, and preferably in the range of about 1.0 to about 10.0, preferably 1.0 to 5.0 centipoise at room temperature.

Other ingredients are also commonly added to ink jet inks. A humectant, or co-solvent, is commonly added to help prevent the ink from drying out or crusting in the orifices of he printhead. A penetrant may also be optionally added to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. A biocide, such as Proxel® GXL from Zeneca Colours may be added at a concentration of 0.05-0.5 weight percent to prevent unwanted microbial growth which may occur in the ink over time. Additional additives which may optionally be present in ink jet inks include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, and defoamers.

The ink jet inks provided by this invention are employed in ink jet printing wherein liquid ink drops are applied in a controlled fashion to an ink receptive layer substrate, by ejecting ink droplets from the plurality of nozzles, or orifices, in a print head of ink jet printers.

Commercially available ink jet printers use several different schemes to control the deposition of the ink droplets. Such schemes are generally of two types: continues stream and drop-on-demand.

In drop-on-demand systems, a droplet of ink is ejected from an orifice directly to a position on the ink receptive layer by pressure created by, for example, a piezoelectric device, an acoustic device, or a thermal process controlled in accordance digital data signals. An ink droplet is not generated and ejected through the orifices of the print head unless it is needed. Ink jet printing methods, and related printers, are commercially available and need not be described in detail.

The following examples further clarify the invention.

EXAMPLES 1-11

Non-gelling ink jet inks, containing a variety of different block copolymers of polyethylene oxide and polypropylene oxide were prepared and then evaluated for image quality. First a millgrind was prepared and then diluted to form the ink.

A typical ink jet ink composition was prepared. First an ink concentrate dispersion was prepared with the following ingredients.

40.0 g Black Pearl 880;

15.0 g OMT;

220.0 g di-ionized water.

The above ingredients were mixed and milled for 6 hours in a high speed mill. The rigid milling media was polymeric. Particle sizes less than 0.1 μm were obtained as measured by a UPA analyzer. The ink concentrate was separated from the milling media and diluted to 7.5% pigment.

An ink jet ink was then formulated from the diluted concentrate by taking an aliquot that contained 4.0 g of pigment and diluted with 5.0 g of diethylene glycol; 5.0 g glycerol and enough de-ionized water to bring the total weight to 100.0 g. The ink was filtered through a 3 micron millipore filter. The ink was then introduced into a high capacity HP 51626A ink cartridge that fits into the HP540 desk printer. Images were printed on Kodak mid-weight glossy paper.

Similar inks were prepared to evaluate a range of block copolymers at a concentration of 1 weight percent, based on the total weight of the ink.

Table 1 summarizes the image quality obtained from the various inks. The block copolymers are designated by their Pluronic® code name dessignations. They are all variations of polyethylene oxide and polypropylene oxide copolymers.

                  TABLE 1                                                          ______________________________________                                                   Block              Ethylene Oxide                                                                          Image                                      Example No. Copolymer MW Weight % Quality                                    ______________________________________                                         Control   none      --                3                                          1 L62 1,630 20 3                                                               2 L64 2,900 40 2-3                                                             3 L101 3,800 10 2-3                                                            4 P104 5,900 40 2-3                                                            5 P105 6,500 50 2-3                                                            6 (Invention) F38 4,700 80 0                                                   7 (Invention) F68 8,400 80 0                                                   8 (Invention) F98 13,000  80 0                                                 9 (Invention) F108 14,600  80 0                                                10 (Invention) 10R8 4,550 80 0                                                 11 (Invention) 17R8 7,000 80 0                                               ______________________________________                                    

Image quality was rated by the degree of image cracking especially in the characters and lines. No cracking was observed in the Dmax areas. The ratings were as follows: 3=large cracks readily visible to naked eye; 2=barely visible; 1=cracks visible only by a 10× magnification; and 0=no cracks.

Table 1 shows that the block copolymers defined for use in the invention provide images that are free of cracks. Each of the copolymers used in the inventive inks were solid and contained 80 weight % ethylene oxide. No gelling of the inks occurred. Copolymers 1-5 of Table 1 were liquid or paste.

Others polymers, such as polyvinylpyrrolidone (MW 15,000) and polyethylene oxide (MW 8,000) were also evaluated in inks prepared as above. The corresponding image quality was rated 1-2 for both polymers.

Images were also printed and evaluated with inks containing quinacridone magenta (pigment red 122) and a commerically available cyan pigment. The images were evaluated for overall quality and cracking. It was found that inks containing no block copolymers defined by this invention were of poor quality due to non-uniform lines. On the other hand images containing the block copolymers according to the invention were of excellent quality, high density and very uniform.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 

We claim:
 1. A liquid ink jet ink wherein said ink is non-gelling and comprises a carrier, a pigment and 0.2 to 5 weight % of a solid block copolymer of ethylene oxide and propylene oxide having a structure selected from the group consisting of: ##STR3## having a number average molecular weight of 4000 to 15,000 and an ethylene oxide content of 80 weight %; and ##STR4## having a number average molecular weight of 4000 to 9000 and an ethylene oxide content of 80 weight %.
 2. The ink of claim 1 comprising from 0.5 to 10.0 weight % of pigment and the carrier is selected from water and water containing miscible organic solvents.
 3. The ink of claim 1 wherein the pigment is selected from the group consisting of cyan, magenta, yellow, black and mixtures thereof.
 4. An ink jet printing method, comprising the steps of:providing an ink jet printer that is responsive to digital data signals; loading the printer with ink receptive substrates; loading the printer with an ink jet ink according to claim 1; and printing on the ink receptive substrates in response to the digital data signals. 